1. Field of the Invention
The present invention relates to a yoke of a universal joint used, for instance, in an automobile steering device for transmitting the movement of a steering wheel shaft to a steering gear.
2. Related Background Art
An automobile steering device is configured, for example, as shown in FIG. 11.
In the figure, 1 is a steering wheel. The movement of the steering wheel 1 is transmitted to a steering gear 4 through a steering wheel shaft 2 and a coupling rod 3. Thus the steering gear 4 steers the wheels.
Normally, the above-mentioned steering wheel shaft 2 and an input shaft 5 of the steering gear 4 cannot be provided on the same straight line. For this reason, the coupling rod 3 is conventionally provided between the aforementioned shafts 2 and 5 to achieve power transmission between the steering wheel shaft 2 and the input shaft 5 by connecting both ends of the coupling rod 3 with the ends of the steering wheel shaft 2 and the input shaft 5 through a pair of universal joints 6 and 6.
The universal joint 6 incorporated in such power transmitting mechanism is conventionally comprised of first and second yokes 7 and 8, each having a forked shape, and a cross shaft 9 for freely linking both yokes 7 and 8 as shown in FIG. 12.
Because of the structure of the section to which the aforementioned universal joint 6 is attached or because of other assembly requirements, there are some cases where a distance shown by "e" in FIG. 12 between the center of the cross shaft 9 and the center of a bolt 10 for fixing the yoke 8 onto the coupling rod 3 or a similar component must be increased.
For such cases, a yoke 11 shown in FIGS. 13 through 16 was produced according to the manufacturing method disclosed in the Japanese Patent application Laid-Open No. 2-35223, and the yoke 11 was attached to the universal joint 6 shown in FIG. 12 in place of the yoke 8.
The yoke 11 is made by press-forming a thick metal plate to obtain adequate rigidity. It has a connecting cylinder part 13 with a gap 12 formed along the full length and with serrations 14 formed in the inside periphery. It also comprises a pair of flanges 15 and 16 with the gap 12 between them at the lower part of the connecting cylinder part 13 (the bottom part shown in FIGS. 13 and 14). Each of the flanges 15 and 16 is formed to have an adequate thickness by folding a part of a metal plate at an angle of 180 degrees. One flange 15 has a tapped hole 17 for screwing the bolt 10, and the other flange 16 has a through hole 18 for inserting the bolt 10, the two holes being coaxial.
When the end of the coupling rod 3 or a similar part is to be connected and fixed within the yoke 11 which has a structure described above and which is assembled to the cross shaft 9 and another yoke 7 to constitute the universal joint 6, the end of the coupling rod 3 is inserted in the connecting cylinder 13 with the bolt 10 kept in a loose condition so that a serrated part formed on the outside periphery of the rod end is engaged with the serrations 14 formed in the inside periphery of the connecting cylinder part 13. Then the coupling rod 3 and the yoke 11 are connected securely by tightening the bolt 10.
The conventional universal joint yoke which has the structure described above is perhaps most notably disadvantageous with regard to its weight and its rigidity against twisting forces.
Specifically, in the conventional yoke 11, the gap 12 is formed along the full length of the long connecting cylinder part 13, and the part of the metal plate is folded back 180 degrees on either side of the gap 12 to form the thick yoke to provide improved rigidity. This means extra metal material used, adding to the weight of the yoke.
Also, since the gap 12 is formed along the full length of the connecting cylinder part 13, the yoke does not necessarily provide adequate rigidity against twisting forces despite the thickness provided on either side of the gap 12. Thus the rigidity may be insufficient for conveying the rotational power through the universal joint 6 incorporating the yoke 11, resulting in poor steering operation of a steering device which incorporates the universal joint 6.